1. Field of the Invention:
This invention relates to a paint composition for under coating which is excellent in corrosion resistance, workability and adhesiveness to a substrate and to a topcoat film.
2. Description of the Prior Art:
Recently there has been an increasing need for improvement in the corrosion resistance of steel plates which are widely employed in automobiles, building materials and appliance materials. For example, the spread of ice melting compositions requires automobiles of improved corrosion resistance, while building materials should be maintenance-free for a prolonged period of time. Known processes for satisfying these requirements include, for example, plating or transforming a matrix steel or undercoating thereof. Examples of paints which have been conventionally employed in the latter method, i.e., undercoating include a composition comprising an organic binder such as rubber, phenol resin or polyester resin and a rust preventive pigment such as zinc chromate or strontium chromate; an organic/inorganic composite paint comprising colloidal silica and an aqueous organic polymer (cf. Japanese Patent Publication No. 34406/1979); an organic/inorganic composite paint having a strengthened bond between an organic resin and a silica component (cf. Japanese Patent Laid-Open No. 199074/1985); and a zinc-rich paint obtained by adding a metal powder such as a zinc powder to an organic binder (cf. Japanese Patent Publication No. 37096/1978 and Japanese Laid-Open Publication No. 152444/1986).
Paint compositions comprising an organic binder and a rust preventive pigment, which are widely employed in electric appliances or building materials, are disadvantageous in that the toxicity of a chrome pigment and the insufficient corrosion resistance thereof require thick-coating in order to achieve a high corrosion resistance, which frequently causes some troubles such as cracking or peeling of a coating film during processing. On the other hand, the corrosion resistance of composite paints comprising an organic polymer and silica is yet insufficient, though it has been considerably improved, and also the alkali resistance and adhesiveness to a topcoat film of these paints are unsatisfactory. In addition, zinc-rich paints are unsuitable to pressing since they contain relatively large metal particles, though the corrosion resistance thereof is highly improved. Thus there is no paint for under coating which satisfies all of these requirements, i.e., satisfactory corrosion resistance, workability and adhesivenss to a substrate and to a topcoat film. Therefore known paints have only limited uses.